1. Field of the Invention
The present invention relates to an ink, an ink jet recording method using the ink, and an ink jet recording apparatus. More specifically, it relates to an ink for ink jet recording for use in the so-called thermal jet recording in which thermal energy is applied to the ink to form droplets of the ink, an ink jet recording method, an ink jet recording unit, an ink cartridge, and an ink jet recording apparatus.
2. Related Background
As ink jet recording techniques, various methods have been heretofore developed, and above all, the so-called thermal jet recording method described in, for example Japanese Patent Application Laid-open No. 54-51837 is useful, because this method uses a high-density multi-nozzle very simply constituted and has the feature that high-quality images can be obtained at a high speed and at very low costs and the other feature that the printing is possible on plain papers having no special coating films.
According to this thermal jet recording method, a liquid to which thermal energy has been applied gives rise to a state change owing to the abrupt increase of volume. By functional force based on this state change, droplets are ejected through an orifice at the tip of a recording head, and these droplets then fly and adhere to a recording material, whereby the recording is carried out.
The principle and the constitution of the thermal jet recording method are very simple, but in order to always reliably realize the characteristics of high image quality and the high-speed printing, many techniques regarding an ink and a head are necessary. In the ink jet recording of the thermal jet type, performances necessary for the ink which can be printed on plain papers are mainly classified into a printing performance, a reliability and an ejection performance.
Important points regarding the printing performance and the reliability are following five items:
(1) Fixing property: The fixing property can be evaluated as a time necessary to apparently dry an ink printed on a recording paper to such a degree that the paper is not soiled with the ink even when rubbing the ink on the paper with a finger. It is preferable that this time is short; PA0 (2) OD: OD is an optical density of an image printed on the recording paper. In general, when a high ratio of a dye that does not penetrate into the paper and remains on the surface of the paper, OD becomes high, which is preferable; PA0 (3) Printed letter quality: The nearer to a true circle a recorded dot is, the more excellent the printed letter quality is. When a conventional ink is printed on plain paper, the printed dot having a rough boundary is obtained owing to an influence of the fiber of the paper; PA0 (4) Crusting property: The crusting property means a property by which the ink evaporates and crusts (brings about clogging) in the head for ink jet recording. The ink which scarcely crusts is preferable; and PA0 (5) Initial ejection property: The initial ejection property means an ejection property of the ink at a time when the ink is initially ejected several times, after the moisture of the ink is evaporated through a nozzle point during the rest of the printing to increase the viscosity of the ink. There may be some cases that an ejecting direction is slanted or the ink is not ejected. PA0 (1) Ejection stability: It is preferred that the size, velocity and direction of ink droplets ejected through an orifice are always uniform without variation; PA0 (2) Ejection velocity: This ejection velocity means a velocity of the ink droplets ejected through the orifice. In generait the ejection velocity is preferably high and usually in the range of about 3 to about 20 m/sec; PA0 (3) Frequency response: After the ink droplets have been ejected, the shorter the time necessary to replenish the ink as much as the amount of the ejected ink is, the higher the frequency response is, which is preferable; and PA0 (4) Life of the head: When a large amount of the recording has been carried out, a heater of the recording head is broken so that an ejection may become impossible sometimes. Alternatively, a deposit (the so-called scorch) is accumulated on the surface of the heater, so that the thermal energy of the heater is not effectively transmitted to the ink inconveniently at times. As a result, ejection performances such as the ejection stability, the ejection velocity and the frequency response deteriorate, or the amount of the ejected ink droplets changes on occasion. The life of the head is represented by the number of ejection pulses during maintence of the initial ejection performance.
These characteristics can be roughly classified into the printing performances of the above-mentioned paragraphs (1) to (3) and the reliability of the abovementioned paragraphs (4) and (5). Most of these characteristics are contrary to each other, and therefore it is difficult to prepare the ink which meets all of the requirements.
For example, the fixing property is contrary to the crusting property. If much attention is paid to the fixing property, the easily evapotable property of the ink is an excellent requirement of the ink, whereas if the crusting property is predominantly considered, the scarcely evapotable property of the ink is an excellent requirement of the ink, which is quite opposite to the former. This is largely inconsistent.
Also between the fixing property and OD, a similar inconsistency is present. In order to improve the fixing property, the penetration of the ink into the recording paper must be increased, but in such a case, the ink inconveniently sinks in a depth direction of the paper. When the ink penetrates into the paper, the fiber on the surface of the paper scatters light to deteriorate OD.
There is also inconsistency between the fixing property and the printed letter quality. That is, the increase of the penetration of the ink into the recording paper leads to the improvement of the fixing property. However, the structure of the paper is nonuniform (due to roughness and fiber), and therefore the degree of the penetration varies. When the highly penetrative ink is used, the degree of the penetration is further variant, so that each dot deviates from the true circle and has a rough and dim boundary.
On the other hand, in the ink jet recording of the thermal jet type, the following four points are required as the ejection performances:
Particularly, in order to prolong the life of the head, the improvement of the ink has been heretofore carried out. For example, The life of the head can be improved by keeping the content of a hearty metal in the ink at a certain concentration or less, as apparently disclosed in Japanese Patent Application Laid-open Nos. 61-113668 (Fe.ltoreq.4 ppm), 61-113669 (Fe+Si.ltoreq.9 ppm), 61-113671 (a divalent metal.ltoreq.20 ppm) and 61-113673 (P.ltoreq.2 ppm).
On the other hand, for example, as disclosed in Japanese Patent Application Laid-open No. 56-42684, it has also been suggested to include a substance used as a film-forming means in the ink to form a film on the surface of the heater, whereby the shock at the times of generation and extinction of bubbles (cavitation) can be relieved. A substance used as the film-forming means in accordance with this suggestion, can include metal-containing compounds such as organic metal chelate compounds, metal salts of an organic acid, metallized dyes and the like.
These suggestions can improve the life of the head to some extent, but they are still poor. The further improvement of the life of the head is desired.